計算機斷層掃描技術（2010高清英文版）Computed Tomography for Technologists.pdf

FM.indd iFM.indd i11/17/2009 12:57:22 PM11/17/2009 12:57:22 PMFM.indd iiFM.indd ii11/17/2009 12:57:24 PM11/17/2009 12:57:24 PMFM.indd iiiFM.indd iii11/17/2009 12:57:24 PM11/17/2009 12:57:24 PMAcquisitions Editor: Pete Sabatini Product Director: Eric Branger Product Manager: Amy Millholen Marketing Manager: Allison Powell Compositor: SPi Technologies Printer: C cm.Includes bibliographical references and index.ISBN 978-0-7817-7751-3 (alk. paper)1. Tomography?Textbooks. 2. Medical technologists. I. Title. [DNLM: 1. Tomography, X-Ray Computed. WN 206 R758c 2011]RC78.7.T6R659 2011616.07 57?dc22 2009045125The publishers have made every effort to trace the copyright holders for borrowed material. If they have inadvertently overlooked any, they will be pleased to make the necessary arrangements at the first opportunity.To purchase additional copies of this book, call our customer service department at (800) 638-3030 or fax orders to (301) 824-7390. International customers should call (301) 714-2324.Visit Lippincott Williams by 2005, it had decreased to just 0.005 seconds! This technologic evolution has opened the door to new and varied uses for CT, from assessing coronary disease to colorectal screening. In some cases, new indications for CT exams may replace other, more invasive procedures; in other situations, such as that of appendicitis, they offer clinicians an alternative approach when a diagnosis is problematic. As the scope and practice of CT expands, so must the knowledge of technologists working in the field. Although the establishment of guidelines and protocols are most often the purview of radiologists, in the course of their work technologists must make myriad decisions that affect the quality of an exam. Such decisions can only be appropriately made if technologists have an adequate foundation in each of the key content areas of CT. The goal of this book is to provide a centralized resource for the CT technologist to gain the knowledge necessary to consistently provide excellent patient care that will result in high quality CT exams. This text will also provide the reader the information necessary to successfully sit for the advanced level certification exam offered by The Ameri- can Registry of Radiologic Technologists (ARRT). This text is also appropriate for radiography students taking a CT course. By identifying three major content categories, the ARRT has provided a framework to allow the assessment of the knowledge and skills that underlie a technologist’s decision-making process. This text is organized in accor-dance to the categories identified by the ARRT. Each major section covers one of the ARRT-designated content areas, namely physics and instrumentation, patient care, and imaging procedures. However, the categorization of topics is far from clear-cut. For instance, a question regard- ing the appropriate type and dose of iodinated contrast for a given procedure could just as easily fall under the cat- egory of patient care as that of imaging procedures. Since topic categories often overlap, many subjects will appear in more than one section of the book, most likely with a slightly different perspective. Using the example above to illustrate, in the section on patient care, contrast media is covered from a global standpoint and includes such things as its characteristics and types. In the section on imaging procedures, the topic of contrast media arises again, this time in regards to why certain agents are preferred for cer- tain types of procedures. Having worked as a technologist for over 20 years, I have approached this text with a technologist’s (not a physicist’s or a radiologist’s) perspective. The focus is on caring for patients and creating quality exams, with just enough physics so that everything makes sense. It is apparent from the content specifications of the advanced level CT certification exam that the ARRT shares my philosophy. Seventy percent of the questions contained on the exam relate to exam protocols and patient care. The ability to accurately identify cross-sectional anat- omy is an important aspect of the technologist’s job and comprises a significant portion of the ARRT certification exam. The anatomy section included in this text is intended only as an introduction to cross-sectional anatomy; the images included should give the reader an idea of the level of anatomic detail with which the technologist is expected to become familiar. Many excellent texts currently exist that provide a full range of cross-sectional images, should the reader wish to continue their studies. Many individuals are looking for a “cookbook” of exam protocols. There are two main problems with creating such a cookbook. First, there are no universally accepted protocols that could be considered the standard of care in the field. Protocols are as varied as the professionals that FM.indd viiFM.indd vii11/17/2009 12:57:29 PM11/17/2009 12:57:29 PMviii Computed Tomography for Technologists: A Comprehensive Textuse them, with adjustments made for the type of patient, the type of scanner, and the preferences of the radiologists. The other main barrier to a cookbook approach is that the rapid advancements in the field of radiology make any such document obsolete before the ink dries. With that caveat, in each main anatomic category I have included a few exam protocols, in many cases ones that we use at the University of Michigan. These are intended as a frame of reference only with the expectation that protocols must constantly evolve to keep up with new developments in the field. The companion website for instructors contains valu- able teaching resources to complement the text. Instructor resources on thePoint include PowerPoint slides for each chapter, an image bank containing all images found in the book, and situational judgment questions. Lois E. Romans, RT, (R)(CT) University of Michigan Health Systems Ann Arbor, MichiganFM.indd viiiFM.indd viii11/17/2009 12:57:30 PM11/17/2009 12:57:30 PMixUser’s GuideThis User’s Guide introduces you to the helpful features of Computed Tomography for Technologists: A Comprehensive Text that enable you to quickly mas- ter new concepts and put your new skills into practice.Chapter features to increase understanding and enhance retention of the material include: Key terms help you focus on the most important concepts as you progress through the chapter. Key Concepts Boxes present important information for readers to remember (exam material). FM.indd ixFM.indd ix11/17/2009 12:57:30 PM11/17/2009 12:57:30 PMRecommended Reading or References provide the opportunity to expand on the knowledge gained from the chapter. Review Questions at the end of each chapter promote a deeper understanding of fundamental concepts by encouraging analysis and application of information presented.Clinical Application Boxes use real-life scenarios to illustrate and explain concepts.x Computed Tomography for Technologists: A Comprehensive TextFM.indd xFM.indd x11/17/2009 12:57:33 PM11/17/2009 12:57:33 PMUser’ s Guide xiCT cross-sectional slices accompanied by shaded diagrams and a reference image are featured in the Cross- Sectional Anatomy section of the book.Examples of Exam Protocols are included for each major anatomical area.FM.indd xiFM.indd xi11/17/2009 12:57:38 PM11/17/2009 12:57:38 PMxii Computed Tomography for Technologists: A Comprehensive TextGlossary in the back of the book defines all the key terms used throughout the text.Student Resources The online student resource center at http://thePoint.lww.com/RomansCT reinforces what you learn in the text. Student resources include full text online, PowerPoint slides, and the protocol tables from the book containing localizer images to guide technologists in setting the scan range of each specific study. See the inside front cover for details on how to access these resources.Instructor Resources The online instructor resources available for use with Computed Tomography for Technologists: A Comprehensive Text include PowerPoints, an image bank, and situational judgment questions.FM.indd xiiFM.indd xii11/17/2009 12:57:41 PM11/17/2009 12:57:41 PMxiiiMatthew G. Aagesen Musculokeletal Fellow University of Michigan Health System Ann Arbor, MIJeffL. Berry, MS, RT (R) (CT) Radiography Program Director Department of Medical Imaging that is, slices looked like the rings of a tree visualized in the cut edge of a log. Therefore, it was Key Terms: spatial resolution · low-contrast resolution · temporal resolution · Z axis · collimators · pixel · voxel · matrix · beam attenuation · low attenuation · high attenuation · linear attenuation coefficient · positive contrast agents · negative contrast agents · Hounsfield units · polychromatic x-ray energy · artifacts · beam-hardening artifacts · cupping artifacts · volume averaging · partial volume effect · raw data · scan data · Image reconstruction · prospective reconstruction · retrospective reconstruction · step-and-shoot scanning · spiral/ helical scanning · multidetector row CT scanning · imaging planes · anatomic position · anterior · ventral · posterior · dorsal · Inferior · caudal · superior · distal · proximal · transverse plane · longitudinal plane · sagittal plane · coronal plane · oblique plane · kinetic energy · gantry · anode · focal spot · tube current · heat capacity · heat dissipation · data acquisition system · view · central processing unit · display processorBASIC PRINCIPLES OF CTBASIC PRINCIPLES OF CTChap01.indd 3Chap01.indd 311/13/2009 9:31:00 AM11/13/2009 9:31:00 AM4 Computed Tomography for Technologists: A Comprehensive Textcommon to refer to older scanning systems as computerized axial tomography, hence the common acronym, CAT scan. Newer model scanners offer options in more than just the transverse plane. Therefore, the word “axial” has been dropped from the name of current CT systems. If the old acronym CAT is used, it now represents the phrase computer-assisted tomography. The historic evolution of CT, although interesting, is beyond the scope of this text. However, for clarity, a few key elements in the development of CT are mentioned here. Although all CT manufacturers began with the same basic form, each attempted to set their scanners apart in the marketplace by adding features and functionality to the existing technology. As each feature was devel- oped, each manufacturer gave the feature a name. For this reason, the same feature may have a variety of dif- ferent names, depending on the manufacturer. For exam- ple, the preliminary image each scanner produces may be referred to as a “topogram” (Siemens), “scout” (GE Healthcare), or “scanogram” (Toshiba). Another well- known example is a method of scanning that, generi- cally, is referred to as continuous acquisition scanning; this method can also be called “spiral” (Siemens), “heli- cal” (GE Healthcare), or “isotropic” (Toshiba) scanning. In many cases, the trade name of the function is more widely recognized than the generic term. This text refers to each function by the name that best describes it or by the term that is most widely used. Once one understands what each operation accomplishes, switching terms to accommodate scanners is simple. CT image quality is typically evaluated using a num- ber of criteria:■ Spatial resolution describes the ability of a system to define small objects distinctly.■ Low-contrast resolution refers to the ability of a sys- tem to differentiate, on the image, objects with similar densities.■ Temporal resolution refers to the speed that the data can be acquired. This speed is particularly impor- tant to reduce or eliminate artifacts that result from object motion, such as those commonly seen when imaging the heart.These aspects of image quality will be explained more fully in Chapter 6.COMPUTED TOMOGRAPHY DEFINEDComputed tomography uses a computer to process infor- mation collected from the passage of x-ray beams through an area of anatomy. The images created are cross-sectional. To visualize CT, the often-used loaf of bread analogy is use- ful. If the patient’s body is imagined to be a loaf of bread, each CT slice correlates to a slice of the bread. The crust of the bread is analogous to the skin of the patient’s body; the white portion of the bread, the patient’s internal organs.The individual CT slice shows only the parts of the anatomy imaged at a particular level. For example, a scan taken at the level of the sternum would show portions of lung, mediastinum, and ribs, but would not show portions of the kidneys and bladder. Computed tomography requires a firm knowledge of anatomy, in particular the under stand- ing of the location of each organ relative to others.Each CT slice represents a specific plane in the patient’s body. The thickness of the plane is referred to as the Z axis. The Z axis determines the thickness of the slices (Fig. 1-1). The operator selects the thickness of the slice from the choices available on the specific scanner. Selecting a slice thickness limits the x-ray beam so that it passes only through this volume; hence, scatter radia- tion and superimposition of other structures are greatly diminished. Limiting the x-ray beam in this manner is accomplished by mechanical hardware that resembles small shutters, called collimators, which adjust the open- ing based on the operator’s selection. The data that form the CT slice are further sectioned into elements: width is indicated by X, while height is indi- cated by Y (Fig. 1-2). Each one of these two-dimensional squares is a pixel (picture element). A composite of thou- sands of pixels creates the CT image that displays on the CT monitor. If the Z axis is taken into account, the result is a cube, rather than a square. This cube is referred to as a voxel (volume element). A matrix is the grid formed from the rows and col- umns of pixels. In CT, the most common matrix size is 512. This size translates to 512 rows of pixels down and 512 columns of pixels across. The total number of pix- els in a matrix is the product of the number of rows and the number of columns, in this case 512 × 512 (262,144). Because the outside perimeter of the square is held con- stant, a larger matrix size (i.e., 1,024 as opposed to 512) ZFIGURE 1?1 The thickness of the cross-sectional slice is referred to as its Z axis.Chap01.indd 4Chap01.indd 411/13/2009 9:31:01 AM11/13/2009 9:31:01 AMBasic Principles of CT 5will contain smaller individual pixels. Each pixel contains information that the system obtains from scanning.BEAM ATTENUATIONThe structures in a CT image are represented by vary- ing shades of gray. The creation of these shades of gray is based on basic radiation principles. An x-ra